Is there a way to draw the terrain map first and then draw the coloured patches on top of it? (preferably without overriding the terrain map with the default map) - or does the way the map is divided into several smaller rectangles somehow prevent this, so that the terrain map always overrides the coloured map or something..?

I suspect drawing only the unexplored areas on top of the terrain map would not look particularly good; regardless, it would require a fairly complicated modification to the addon to prevent it from drawing overlays you already know (right now it simply draws them under the existing overlays of the world map).

If you're only interested in knowing which areas remain unexplored, I wrote an addon for someone some time ago which unmistakably marks them on your map.

Yea, OK.. that helped. I mean, that's kind of what "sagerobot" asked about.
Or if I may not need Mapster's FogLight at some point (which is unlikely for other reasons, but anyway), then I'd still see the unexplored areas better. But the thing is, I usually stick with the "All Terrain" mode all the time (and why wouldn't I, as it looks so much nicer), so I never see the colours (neither Foglight's nor Mapster's).
Now, when I set terrainAlpha to 80%, that's closer to what I was getting at.
Is there a way to draw the terrain map first and then draw the coloured patches on top of it? (preferably without overriding the terrain map with the default map) - or does the way the map is divided into several smaller rectangles somehow prevent this, so that the terrain map always overrides the coloured map or something..?

Mapster's colouration is barely visible with the No Terrain mode, even less with the Hybrid mode, and not visible at all with the All Terrain mode. It would be nicer if the All Terrain map would somehow not be drawn over Mapster's colouration, and instead have the coloured patches on it.

If you open Foglight.lua, at the very top of the file you can set the RGB (amount of red green and blue) color of the overlays that it's filling in.

By default they're simply darker than normal, using "1, 0, 0" would make them red.

What I was referring to, was about the combination of these two:
(1) http://postimg.org/image/gk0449c65/full/
(2) http://postimg.org/image/cq6nv3uu5/full/
which could result in something like this:
(3) http://postimg.org/image/myz50xivx/full/
Mapster's colouration is barely visible with the No Terrain mode, even less with the Hybrid mode, and not visible at all with the All Terrain mode. It would be nicer if the All Terrain map would somehow not be drawn over Mapster's colouration, and instead have the coloured patches on it. (So, not quite like in my third picture, but with the minimap terrain in place of the default map underneath the coloured patches.)
Maybe this could be achieved by reducing the terrain overlay's strata? I don't know..

Is it possible to turn off the map reveal? I am trying to see the places I need to visit but cant seem to find an option to turn off.

The "Disabled" mode should be what you're looking for. It's just that if you have already revealed most of the map, the few areas that are still missing may be very hard to see on the map, as the map's background is often barely distinguishable from the surrounding (revealed) areas.

This also depends on which other map-related addons you may intend to use, e.g. you could use something like Mapster and its FogClear option which, with the "Disabled" mode, will colour the missing areas correctly, but it won't do much good with any other modes.

Addendum: Just to be clear, by "Disabled" mode, I mean the option provided with the little drop-down menu on the map directly. (I did not mean disabling Foglight altogether.)